Conventional electrostatographic toner powders are made up of a binder polymer and other ingredients, such as pigment and a charge control agent, that are melt blended on a heated roll or in an extruder. The resulting solidified blend is then ground or pulverized to form a powder. Inherent in this conventional process are certain drawbacks. For example, the binder polymer must be brittle to facilitate grinding. Improved grinding can be achieved at lower molecular weight of the polymeric binder. However, low molecular weight binders have several disadvantages; they tend to form toner/developer flakes; they promote scumming of the carrier particles that are admixed with the toner powder for electrophotographic developer compositions; their low melt elasticity increases the off-set of toner to the hot fuser rollers of the electrophotographic copying apparatus, and the glass transition temperature (Tg) of the binder polymer is difficult to control. In addition, grinding of the polymer results in a wide particle size distribution. Consequently, the yield of useful toner is lower and the manufacturing costs are higher. Also the toner fines accumulate in the developer station of the copying apparatus and adversely affect the developer life.
Another problem with conventional electrostatographic toner powders is that image quality is limited by the size of the toner particles. Smaller toner particles result in improved image quality but such small toner particles are expensive to manufacture by conventional grinding and pulverizing techniques and suffer from the issues listed above.
The preparation of toner polymer powders from a preformed polymer by the chemically prepared toner process such as the “Evaporative Limited Coalescence” (ELC) offers many advantages over the conventional grinding method of producing toner particles. In this process, polymer particles having a narrow size distribution are obtained by forming a solution of a polymer in a solvent that is immiscible with water, dispersing the solution so formed in an aqueous medium containing a solid colloidal stabilizer and removing the solvent. The resultant particles are then isolated, washed and dried.
In the practice of this technique, polymer particles are prepared from any type of polymer that is soluble in a solvent that is immiscible with water. Thus, the size and size distribution of the resulting particles can be predetermined and controlled by the relative quantities of the particular polymer employed, the solvent, the quantity, and the size of the water insoluble solid particulate suspension stabilizer, typically silica or latex, and the size to which the solvent-polymer droplets are reduced by mechanical shearing using rotor-stator type colloid mills, high pressure homogenizers, agitation etc.
Limited coalescence techniques of this type have been described in numerous US Patents pertaining to the preparation of electrostatic toner particles because such techniques typically result in the formation of polymer particles having a substantially uniform size distribution. Representative limited coalescence processes employed in toner preparation are described in U.S. Pat. Nos. 4,833,060 and 4,965,131 to Nair et al., incorporated herein by reference for all that they contain.
Limited coalescence techniques can produce smaller toner particles with narrower size distributions than grinding and pulverizing. As mentioned previously, these smaller toner particle sizes result in improved image quality. However, even better image quality can be achieved by pigment particles suspended or dispersed in a liquid carrier. The liquid carrier is typically a nonconductive dispersant, to avoid discharging the latent electrostatic image. Liquid toners contain pigments that are typically smaller than dry toner particles. Because of their small particle size, ranging from about 5 microns to sub-micron, liquid toners are capable of producing very high-resolution toned images.
A typical liquid toner composition generally comprises a visual enhancement additive (for example, a colored pigment particle) and a polymeric binder. The choice of pigments is limited to those which are compatible with the electrophotographic process, especially with regard to surface charging properties. Thus, there are pigments which provide desirable color tones but are unsuitable for use as liquid toners. The polymeric binder fulfills functions both during and after the electrophotographic process. With respect to processability, the character of the binder impacts charging and charge stability, flow, and fusing characteristics of the toner particles. These characteristics are important to achieve good performance during development, transfer, and fusing. After an image is formed on the final receptor, the nature of the binder (e.g. glass transition temperature, melt viscosity, molecular weight) and the fusing conditions (e.g. temperature, pressure, and fuser configuration) impact durability (e.g. blocking and erasure resistance), adhesion to the receptor, gloss, and the like.
Polymeric binder materials suitable for use in liquid toner particles typically exhibit glass transition temperatures of about −24° C. to 55° C., which is lower than the range of glass transition temperatures (50° C. to 100° C.) typical for polymeric binders used in dry toner particles. In particular, some liquid toners are known to incorporate polymeric binders exhibiting glass transition temperatures (Tg) below room temperature (25° C) in order to rapidly self fix, e.g., by film formation, in liquid electrophotographic imaging process; see e.g. U.S. Pat. No. 6,255,363. However, such liquid toners are also known to exhibit inferior image durability resulting from the low Tg (e.g. poor blocking and erasure resistance) after fusing the toned image to a final image receptor.
There remains a long standing need for dry toners which have the superior image quality comparable to liquid toners while maintaining the convenience, ease of handling and excellent image durability associated with dry toners. In addition, there is a need for an electrophotographic toner which provides high quality images using any pigment regardless of its surface charge.
Microcapsule toners consisting of a polymeric shell and core of hydrophobic liquid and pigment are known. See, for instance, Japanese Kokai 05-313401, Japanese Kokai 56-142539 and Japanese Kokai 04-296868. These microcapsules have a single liquid domain contained by a thin shell of polymer and are subject to premature breakage during manufacture, for example during sieving or surface treatment, and during the imaging process, resulting in premature release of the entire liquid core. It is difficult to make microcapsules which survive mechanical forces during manufacture and during the imaging process, yet are fragile enough to rupture when desired.
Another issue with microcapsule toners is that the shell frequently doesn't have the properties of polymeric binders as described above. In some cases the polymeric shell does not fuse to the receptor sheet.
Japanese Kokai 63-147171 discloses an electrophotographic developer obtained by impregnating a small-diameter sponge with a liquid developer. The liquid developer is pressed in a development area to discharge the liquid from the sponge. Thus, the small-diameter sponge is only a carrier and is not part of the ink. It does not fuse onto the receptor sheet forming part of the image.
There is a need to improve the image quality of dry electrophotographic toner imaging systems. The present invention achieves this objective by providing an ink having discrete particles made from a polymeric binder typical for dry toners containing a liquid phase of an oil and a very small sized pigment typical for liquid toners. The liquid phase is contained in multiple domains within particles of the polymeric binder which means that the entire liquid content of the particle will not be released by premature breakage.
An object of the present invention is to provide an ink polymer particle having a solid phase and a liquid phase that includes oil and pigment.
A further object of the present invention is to provide an ink polymer particle that does not release the liquid prematurely.
A further object of the present invention is to provide a dry toner particle with improved image quality.
A still further object of the present invention is to provide a dry toner particle that can use any pigment.